A similar device for the alternative detection of positively and negatively charged ions at the output of a mass spectrometer is presented in German Patent 2,825,760. With this device, a rapid switch-over can be performed for the alternative measurement of positively and negatively charged ions, since all that is required is to reverse the polarity of the voltage at the conversion dynode or conversion electrode
Spectra of an identical sample of positively and negatively charged ions can be recorded alternately with such a device.
As a rule, spectrometers for analysing positively and negatively charged ions consist of a secondary electron multiplier, whose first dynode separates ions coming from a mass spectrometer. In this case, the first dynode is connected to a potential in the kilovolt range, so that the ions emerging from the mass spectrometer are accelerated through the electric field, and can trigger secondary processes as a result of the kinetic energy absorbed.
It is known from the International Journal of Mass Spectrometry and Ion Processes, 69 (1986), pages 233-237 that when impinging on surfaces organic ions having a kinetic energy of a few keV can, apart from electron emission, also cause ion emission. This phenomenon is used in various mass spectrometers for the purpose of detecting negative ions by firstly directing the latter onto a so-called conversion dynode, from which positively charged secondary dary ions of different origin are then accelerated onto the secondary electron multiplier, in order to liberate electrons there in a known way. Such methods for analysing ions of high mass are presented and described in the printed publications DE 2,825,760, U.S. Pat. No. 4,423,324 and U.S. Pat. No. 4,810,882.
Moreover, the literature contains numerous papers on SID (Surface Included Dissociation), a technique in which complexes of organic primary ions with surfaces of characteristic secondary ions (fragments), the subsequent mass analysis of which is used for the structural resolution of the primary ions, are produced by impact processes.
All previously known mass spectrometric detector arrangements with so-called postacceleration have the aim of enhancing the detection sensitivity for high masses through an improved secondary electron yield. Normally, for this purpose the ions separated in the mass spectrometer are accelerated to a typically 20-30 keV before impinging on the electron-liberating surface. This electrode, which is also termed a conversion dynode, is connected such that in the case of negative primary ions, positive secondary ions, and in the case of positive primary ions, negative secondary ions/electrons are accelerated towards the secondary electron multiplier (SEM). If a photocathode is further connected upstream of the secondary electron multiplier, negative primary electrons are accelerated onto an electrode, from which electrons are then liberated, since photocathodes react only in this way.
So far, the secondary ions have been accelerated directly onto the first dynode or the front side of a so-called microchannel plate in order to liberate electrons there.
In such a design, when there is a change in the potential of this electrode the energy of the secondary particles entering the secondary electron multiplier is also automatically changed as well, or its polarity is determined. This detector arrangement is, however, only to be used for a limited field of application.
Hillenkamp laser desorption has for the first time made accessible an analysable mass range for biomolecules above 50,000 daltons Until recently, there was no method which would have permitted proteins with a molecular weight of 200 to 300,000 daltons to be ionised intact.
Furthermore, it has been established in extensive experiments that when such macromolecules having a kinetic energy of a few 10 keV impinge ions of both polarities are ejected. The emission rate of electrons is comparatively very low. Thus, for these macromolecular ions the detection signal is mainly caused by secondary ions and not by secondary electrons which are alone responsible for the detection of the primary ions, are emitted from the conversion dynodes normally used. In this primary step, electrons are of only secondary importance for detection.
However, for primary ions larger than 3,000 daltons and in the range of kinetic energy from 10 to 50 keV none of the abovementioned detector devices has yet delivered a satisfactory yield of secondary electrons for a satisfactory signal.